A conventional refractor is an optical tool used by optometrists to help measure refractive errors in a person's vision. The person looks through the refractor at an image, as their optometrist selects lenses to place in the person's line of sight. The lenses may be made of glass or optical grade plastic. The refractor holds the lenses in a housing, and it helps organize the lenses in a way that lets the optometrist know which lense(s) the person is looking through to see the image, and what the refractive characteristics are of the chosen lenses. The person looks through the chosen lenses, and tells the optometrist whether a given choice of lenses in a sequence of lens choices makes the image appear more focused or less focused—the optometrist may repeatedly ask if the view through chosen lenses is “better or worse?” Thus, vision testing with a refractor is subjective, and proceeds in discrete lens-combining steps under the direct supervision of the optometrist. Refractors of this general type have been used for decades. For example, the U.S. trademark PHOROPTOR for eye-testing instruments for testing the refractive media and motor muscles of the eye was registered in 1922 (PHOROPTOR is a mark of Leica Microsystems Inc.).
More recently, autorefractors have been created. An autorefractor also includes lenses organized in a housing, but the choice of lenses to place in the person's line of sight is at least partially made by a computer program within the autorefractor. The person takes a seat and places their chin on a rest. One eye at a time, the person looks at a picture inside the autorefractor. The picture moves in and out of focus as the autorefractor places different lenses in the person's line of sight, while the autorefractor checks to see whether the corresponding image is in focus on the person's retina. Several readings can be automatically taken and averaged. No feedback is needed from the person; the testing is objective. However, it still proceeds in discrete lens combination steps with a relatively small number of lenses. With an autorefractor, an approximate measurement of a person's refractive error can be made quickly and automatically. However, it is not always as accurate as more labor-intensive and time-consuming tests, so an autorefractor measurement may be used merely as the starting point for the optometrist to perform a subjective refraction test using lenses in a manually controlled refractor as described above.
In short, a conventional refractor used for vision testing, such as a conventional autorefractor or a PHOROPTOR refractor, operates by changing the refraction in a beam of a sharp-edged source image until the image is perceived by the patient to be in focus, and then noting what change in refraction had to be added by the lenses to the patient's own optical refraction to obtain perceived focus. That added refraction change—the best-working lens combination—can then be replicated or at least approximated by eyeglasses or contact lenses, or by refractive surgery such as LASIK surgery.
It should be noted that the foregoing background was written in hindsight, with the present invention in mind. Thus, the particular cluster of ideas discussed in this background, or their characterization here, would not necessarily have been obvious to someone who did not have the benefit of familiarity with the invention.